Iron golf club

ABSTRACT

An iron golf club having high compatibility of a head and a shaft and good grasp of a ball is provided. An iron golf club includes a head, a shaft and a grip. A head gravity center distance that is a distance between a center of gravity of the head and an axis of the shaft is Db (mm), and a shaft torque of the shaft is Ts (°). Here, Db is 37 mm or more. In addition, Ts is less than 4.0°. Further, Db/Ts is 9.7 or more. The iron golf club has good grasp and excellent driving distance performance.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority of Japan patent application serial no. 2017-135551, filed on Jul. 11, 2017. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification.

BACKGROUND Technical Field

The disclosure relates to an iron golf club.

Description of Related Art

An iron golf club differs from a wood golf club in many ways. A shaft for an iron is used for an iron. Japanese Patent No. 5439259 discloses a shaft set used in an iron set. In the shaft set, bending rigidity of the shafts is changed for each number.

[Patent Document 1] Japanese Patent No. 5439259

SUMMARY

As a result of intensive study by the inventor(s), new knowledge about compatibility between an iron head and a shaft has been obtained.

The disclosure provides an iron golf club having high compatibility of a head and a shaft and good grasp of a ball.

In a certain aspect, an iron golf club includes a head, a shaft and a grip. A head gravity center distance that is a distance between a center of gravity of the head and an axis of the shaft is Db (mm). A shaft torque of the shaft is Ts (°). Db may be 37 mm or more. Ts may be less than 4.0°. Db/Ts may be 9.7 or more.

A distance from the center of gravity of the shaft to a tip end of the shaft is Lg and a length of the shaft is Ls. In another aspect, Lg/Ls may be 0.50 or more.

A club weight is We (grams) and a club length is Lc (meters). In another aspect, Wc×Lc may be 360 (μm) or less.

In another aspect, the shaft may be formed of a plurality of fiber reinforcement layers. The shaft may have a bias layer of a tip portion and a bias layer of a butt portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a golf club of an embodiment.

FIG. 2 is a front view of a head.

FIG. 3 is a developed view showing an example of a laminated constitution of a shaft.

FIG. 4 is a schematic view showing a measurement method of a shaft torque.

DESCRIPTION OF THE EMBODIMENTS

According to an embodiment, it is possible to obtain an iron golf club having excellent compatibility of a shaft and a head and good grasp of a ball.

Hereinafter, an embodiment will be appropriately described in detail with reference to the accompanying drawings.

FIG. 1 shows a golf club 2 of an embodiment. The club 2 is an iron golf club. A loft angle of the iron golf club is generally 15 degrees or more and 70 degrees or less. Further, in this application, the loft angle is a real loft angle. The real loft angle is a loft angle with respect to a shaft axis.

The golf club 2 has a head 4, a shaft 6 and a grip 8. The head 4 is attached to a tip section of the shaft 6. The head 4 is an iron type golf club head. The grip 8 is attached to a rear end portion of the shaft 6.

The shaft 6 has a tip end Tp, a butt end Bt, a shaft length Ls and a center of gravity Gs of the shaft. The center of gravity Gs of the shaft is a center of gravity of the shaft 6 as a single body. A distance shown by both of arrows Lg in FIG. 1 is a distance from the tip end Tp to the center of gravity Gs of the shaft. The shaft length Ls and the distance Lg are measured along an axis Z of the shaft 6.

The shaft length Ls of the shaft for an iron is generally 860 mm or more and 991 mm or less.

FIG. 2 is a view showing the head 4 from the front of a striking edge. The head 4 has a sole 10, a striking edge 12 and a hosel 14. The hosel 14 has a hosel hole 16. A score line gv is formed on the striking edge 12. The striking edge 12 is a flat surface except the score line gv.

The head 4 has a center of gravity Gh. The center of gravity Gh of the head is a center of gravity of the head 4 as a single body. The striking edge 12 has a sweet spot SS. The sweet spot SS is an intersection between a straight line passing through the center of gravity Gh of the head perpendicular to the striking edge 12 and the striking edge 12. In FIG. 2, the sweet spot SS and the center of gravity Gh of the head overlap.

In this application, the head gravity center distance is defined. The head gravity center distance is a distance between a shaft axis Z and the center of gravity Gh of the head when seen in a front view. The front view is a projection view in which a subject is projected to a plane parallel to a face side. A projection surface thereof is disposed in front of the face side. A direction of the projection is a direction perpendicular to the face side. FIG. 2 is a front view of the head 4. The head gravity center distance Db is shown in FIG. 2. Further, the shaft axis Z coincides with a centerline of the hosel hole 16.

In this application, a shaft torque is defined. The shaft torque is an index showing twist rigidity of the shaft 6. In this application, the shaft torque is represented by Ts. A measurement method of the shaft torque will be described below.

The shaft 6 is a so-called carbon shaft. In an embodiment, the shaft 6 is obtained by curing a prepreg sheet. In the prepreg sheet, fibers are oriented in substantially one direction. The prepreg in which such fibers are oriented in substantially one direction is also referred to as a UD prepreg. “UD” stands for unidirectional. A prepreg other than the UD prepreg may be used. For example, the fibers contained in the prepreg sheet may be woven.

The prepreg sheet has a fiber and a resin. The resin is also referred to as a matrix resin. Typically, the fiber is a carbon fiber. Typically, the matrix resin is a thermosetting resin.

The shaft 6 is manufactured by a so-called sheet winding method. In the prepreg, the matrix resin is in a half-hardened state. The shaft 6 is obtained by winding and curing the prepreg sheet.

In addition to an epoxy resin, the matrix resin of the prepreg sheet is obtained using a thermosetting resin, a thermoplastic resin, or the like, other than the epoxy resin. In view of shaft strength, the matrix resin is an epoxy resin in an embodiment.

Hereinafter, a configuration of the shaft 6 will be described in detail.

FIG. 3 is an example of a developed view (a laminated constitution view) of the prepreg sheet that constitutes the shaft 6.

The shaft 6 is constituted by a plurality of sheets. The shaft 6 is constituted by twelve sheets from a first sheet s1 to a twelfth sheet s12. The developed view sequentially shows the sheets that constitute the shaft from the inside of the shaft in a radial direction. These sheets are sequentially wound from the sheet disposed on the upper side in the developed view. In the developed view, a leftward/rightward direction in the drawings coincides with an axial direction of the shaft. In the developed view, a right side in the drawings is the tip end Tp side of the shaft. In the developed view, a left side in the drawings is the butt end Bt side of the shaft.

The developed view also shows a disposition of the sheets in the shaft axis direction as well as a winding sequence of the sheets. For example, in FIG. 3, an end of the ninth sheet s9 is disposed at the tip end Tp. For example, in FIG. 3, an end of the fifth sheet s5 is disposed at the butt end Bt.

In this application, the term “layer” and the term “sheet” are used. “Layer” is the term used after winding. On the other hand, “sheet” is the term used before winding. A “layer” is formed by winding a “sheet.” That is, the wound “sheet” forms the “layer.” In addition, in this application, the same reference character is used for a layer and a sheet. For example, the layer formed by the sheet s1 is a layer s1.

The shaft 6 has a straight layer, a bias layer and a hoop layer. In the developed view of this application, for each sheet, an orientation angle θf of the fiber is shown. The orientation angle θf is an angle with respect to the shaft axis direction.

The shaft 6 has six bias layers. These bias layers constitute three bias layer sets. That is, the embodiment of FIG. 3 has a first bias layer set b1, a second bias layer set b2 and a third bias layer set b3.

The first bias layer set b1 is constituted by a bias layer s1 and a bias layer s2. The fibers are inclined between the bias layer s1 and the bias layer s2 in opposite directions. The second bias layer set b2 is constituted by a bias layer s3 and a bias layer s4. The fibers are inclined between the bias layer s3 and the bias layer s4 in opposite directions. The third bias layer set b3 is constituted by a bias layer s10 and a bias layer s11. The fibers are inclined between the bias layer s10 and the bias layer s11 in opposite directions.

The sheet shown as “0°” constitutes the straight layer. The sheet that constitutes the straight layer is also referred to as the straight sheet.

The straight layer is a layer having the angle θf that is substantially 0°. Due to an error or the like in winding, conventionally, the angle θf is not exactly 0°.

Conventionally, in the straight layer, an absolute angle θa is 10° or less. The absolute angle θa is an absolute value of the orientation angle θf For example, the fact that the absolute angle θa is 10° or less means that the angle θf is −10° or more and +10° or less.

In the embodiment of FIG. 3, the straight sheet is the sheet s5, the sheet s6, the sheet s8, the sheet s9 and the sheet s12.

In the bias layer, there is a high correlation between twist rigidity and twist strength of the shaft. In an embodiment, the bias sheet includes two sheets in which orientations of the fibers are inclined in opposite directions. In view of the twist rigidity, the absolute angle θa of the bias layer is preferably 15° or more, more preferably 25° or more, and even more preferably 40° or more. In view of the twist rigidity and the bending rigidity, the absolute angle θa of the bias layer is preferably 60° or less and more preferably 50° or less.

As described above, in FIG. 3, for each sheet, the angle θf is shown. The bias sheet s1 and the bias sheet s2 that constitute the first bias layer set b1 have inclination angles of fibers that are opposite to each other. A positive (+) and a negative (−) in the angle θf show that the fibers of the bias sheet are inclined in opposite directions. The second bias layer set b2 and the third bias layer set b3 are also similar to the first bias layer set b1.

As shown by two diagonal lines illustrated in FIG. 3, an inclination direction of the fiber of the sheet s2 is equal to the inclination direction of the fiber of the sheet s1. However, the sheet s2 is turned inside out and adhered to the sheet s1. As a result, the angle θf of the sheet s1 and the angle θf of the sheet s2 are opposite to each other.

The shaft 6 has a hoop layer. In the shaft 6, the hoop layer is the layer s7. In the shaft 6, the sheet that constitutes the hoop layer is the seventh sheet s7. The sheet that constitutes the hoop layer is also referred to as a hoop sheet. In an embodiment, the hoop layer is a carbon fiber reinforcement layer.

In an embodiment, the absolute angle θa in the hoop layer is substantially 90° with respect to the shaft axis. However, due to an error or the like in winding, the orientation of the fiber may not be exactly 90° with respect to the shaft axis direction. Conventionally, in the hoop layer, the angle θf is −90° or more and −80° or less, or 80° or more and 90° or less. In other words, conventionally, in the hoop layer, the absolute angle θ is 80° or more and 90° or less.

The number of plies (the number of windings) of one sheet is not limited. For example, when the number of plies of the sheet is one, the sheet is wound one turn in a circumferential direction. For example, when the number of plies of the sheet is two, the sheet is wound two turns in the circumferential direction. For example, when the number of plies of the sheet is 1.5, the sheet is wound 1.5 turns in the circumferential direction. When the number of plies of the sheet is 1.5, the sheet forms one layer at a position of 0 to 180° in the circumferential direction, and forms two layers at a position of 180° to 360° in the circumferential direction.

While not shown, the prepreg sheet before use is sandwiched between the cover sheets. Conventionally, the cover sheet is a release paper and a resin film. The prepreg sheet before use is sandwiched between the release paper and the resin film. A release paper is adhered to one surface of the prepreg sheet, and the resin film is adhered to the other surface of the prepreg sheet. In the following description, a surface to which the release paper is adhered is also referred to as “a surface on the side of the release paper,” and a surface to which the resin film is adhered is also referred to as “a surface on the side of the film.”

The developed view in this application is a view in which a surface on the side of the film is a front side. That is, in FIG. 3, a front side in the drawing is a surface on the side of the film, and a back side in the drawing is a surface on the side of the release paper.

In winding the prepreg sheet, first, the resin film is exfoliated. As the resin film is exfoliated, a surface on the side of the film is exposed. The exposed surface has stickiness (adhesiveness). The stickiness is due to the matrix resin. That is, since the matrix resin is in a half-hardened state, adhesiveness is exhibited. An edge portion of the surface on the side of the exposed film is also referred to as a cutoff edge portion. Next, the cutoff edge portion is adhered to a winding object. Adhesion of the cutoff edge portion is smoothly performed by the adhesiveness of the matrix resin. The winding object is a mandrel or a winding substance obtained by winding another prepreg sheet on the mandrel. Next, the release paper is exfoliated. Next, the winding object is rotated, and the prepreg sheet is wound on the winding object. In this way, after the resin film is exfoliated and the cutoff edge portion is adhered to the winding object, the release paper is exfoliated. Wrinkles or winding errors of the sheet are minimized due to this sequence. Since the sheet to which the release paper is adhered is supported by the release paper, wrinkles cannot easily occur. The release paper has higher bending rigidity than the resin film.

In the embodiment of FIG. 3, a part of the sheet is a united sheet. The bias sheet is wound as a united sheet. The united sheet is formed by attaching two or more sheets. The first bias layer set b1 is a united sheet obtained by uniting the sheet s1 and the sheet s2. The second bias layer set b2 is a united sheet obtained by uniting the sheet s3 and the sheet s4. The third bias layer set b3 is a united sheet obtained by uniting the sheet s10 and the sheet s11.

As described above, the sheet and the layer are classified depending on the orientation angle of the fibers. Further, the sheet and the layer are classified depending on an axial length of the fibers.

In this application, a layer disposed in substantially the entire region in the axial direction is referred to as a full length layer. In this application, a sheet disposed in substantially the entire region in the axial direction is referred to as a full length sheet. The wound full length sheet forms the full length layer.

A region from a point separated 20 mm from the tip end Tp in the axial direction to the tip end Tp is referred to as a first region. In addition, a region from a point separated 100 mm from the butt end Bt in the axial direction to the butt end Bt is referred to as a second region. An influence exerted on performance of the shaft by the first region and the second region is restrictive. In view of this, the full length sheet may not be present in the first region and the second region. In an embodiment, the full length sheet extends from the tip end Tp to the butt end Bt. In other words, the full length sheet provided in an embodiment is disposed in the shaft axis direction as a whole.

In this application, a layer partially disposed in the shaft axis direction is referred to as a partial layer. In this application, a sheet partially disposed in the shaft axis direction is referred to as a partial sheet. The wound partial sheet forms the partial layer. An axial length of the partial sheet is smaller than the axial length of the full length sheet. In an embodiment, the axial length of the partial sheet is a half or less of the full length of the shaft.

In this application, the full length layer that is a straight layer is referred to as a full length straight layer. In the embodiment of FIG. 3, the full length straight layer includes the layer s6 and the layer s8. The full length straight sheet includes the sheet s6 and the sheet s8. In an embodiment, the full length straight layer is a carbon fiber reinforcement layer.

In this application, the full length layer that is a bias layer is referred to as a full length bias layer. In the embodiment of FIG. 3, the full length bias layer includes the layer s1 and the layer s2. In an embodiment, the full length bias layer is a carbon fiber reinforcement layer.

In this application, the full length layer that is a hoop layer is referred to as a full length hoop layer. In the embodiment of FIG. 3, the full length hoop layer is the layer s7.

In this application, the partial layer that is a straight layer is referred to as a partial straight layer. In the embodiment of FIG. 3, the partial straight layer includes the layer s5, the layer s9 and the layer s12.

In this application, the partial layer that is a bias layer is referred to as a partial bias layer. In the embodiment of FIG. 3, the partial bias layer includes the layer s3, the layer s4, the layer s10 and the layer s11.

In this application, the term “a partial butt layer” is used. A straight layer of the butt portion is exemplified as the partial butt layer. In the embodiment of FIG. 3, the straight layer of the butt portion is the layer s5.

A bias layer of the butt portion is exemplified as another partial butt layer. In the embodiment of FIG. 3, the bias layer of the butt portion includes the layer s3 and the layer s4. As described above, the layer s3 and the layer s4 constitute the second bias layer set b2. In an embodiment, the bias layer of the butt portion is a carbon fiber reinforcement layer.

An axial distance between the partial butt layer (the partial butt sheet) and the butt end Bt is preferably 100 mm or less, more preferably 50 mm or less, and even more preferably 0 mm. In the embodiment, in the entire partial butt layer, the distance is 0 mm.

In this application, the term “a partial tip layer” is used. A straight layer of the tip portion and a bias layer of the tip portion are exemplified as the partial tip layer. In the embodiment of FIG. 3, the straight layer of the tip portion includes the layer s9 and the layer s12. The bias layer of the tip portion includes the layer s10 and the layer s11. As described above, the layer s10 and the layer s11 constitute the third bias layer set b3. In an embodiment, the bias layer of the tip portion is a carbon fiber reinforcement layer.

An axial distance between the partial tip layer (the partial tip sheet) and the tip end Tp is preferably 40 mm or less, more preferably 30 mm or less, even more preferably 20 mm or less, and most preferably 0 mm. In the embodiment, in the partial tip layer as a whole, the distance is 0 mm.

[2. Summary of Process of Manufacturing Shaft]

The following is a summary of a process of manufacturing a shaft.

(1) Shearing Process

In a shearing process, a prepreg sheet is sheared in a desired shape. The sheets shown in FIG. 3 are cut out through the process.

The shearing may be performed by a cutting machine. The shearing may be performed by a manual operation. In the case of the manual operation, for example, a cutter knife is used.

(2) Adhering Process

In an adhering process, the above-mentioned united sheet is fabricated.

In the adhering process, heating or pressing may also be used. In an embodiment, the heating and the pressing are used in combination. In a winding process to be described below, during a winding operation of the united sheet, a deviation between the sheets may occur. The deviation decreases winding accuracy. The heating and the pressing improve an adhesive force between the sheets. The heating and the pressing minimize the deviation between the sheets in the winding process.

(3) Winding Process

In the winding process, a mandrel is prepared. A typical mandrel is formed of a metal. A releasing agent is applied to the mandrel. Further, a resin having adhesiveness is applied to the mandrel. The resin is also referred to as a tacking resin. The sheared sheet is wound on the mandrel. An end portion of the sheet can be easily adhered to the mandrel by the tacking resin.

The sheets are wound in sequence as shown in the developed view. The sheet disposed at an upper side in the developed view is wound first. The adhered sheets are wound as the united sheet.

A wound body is obtained by the winding process. The wound body is obtained by winding a prepreg sheet outside the mandrel. The winding is accomplished by, for example, rolling a winding object on a flat surface. The winding may be performed by a manual operation or may be performed by a machine. The machine is referred to as a rolling machine.

(4) Tape Wrapping Process

In a tape wrapping process, a tape is wound on an outer circumferential surface of the wound body. The tape is also referred to as a wrapping tape. The tape is wound while applying a tensile force. A pressure is applied to the wound body by the tape. The pressure reduces voids.

(5) Curing Process

In a curing process, the wound body after the tape wrapping is performed is heated. A matrix resin is cured by the heating. The matrix resin is fluidized temporarily in the middle of the curing. According to the fluidization of the matrix resin, air between the sheets or in the sheet is discharged. Due to a pressure (a fastening force) of the wrapping tape, discharge of the air is accelerated. A cured and laminated body is obtained by the curing.

(6) Process of Extracting Mandrel and Process of Removing Wrapping Tape

After the curing process, a process of extracting the mandrel and a process of removing the wrapping tape are performed. In view of improving efficiency of the process of removing the wrapping tape, the process of removing the wrapping tape is performed after the process of extracting the mandrel according to an embodiment.

(7) Process of Cutting both Ends

In this process, both ends portions of the cured and laminated body are cut. An end surface of the tip end Tp and an end surface of the butt end Bt are flat due to the cutting.

Further, for the convenience of understanding, in the developed view of this application, a sheet having a dimension after cutting of both ends is shown. In actuality, in the dimension upon shearing, cutting of both ends is considered. That is, in actuality, a dimension of a portion at which both ends are cut is added, and shearing is performed.

(8) Polishing Process

In this process, a surface of the cured and laminated body is polished. A spiral concavo-convex shape is present on the surface of the cured and laminated body. The concavo-convex shape is a trace of the wrapping tape. The concavo-convex shape disappears and the surface is smoothed through the polishing. According to an embodiment, in the polishing process, polishing of the entire body and polishing of a tip portion are performed.

(9) Painting Process

The cured and laminated body after the polishing process is painted.

The shaft 6 is obtained through the above-mentioned processes.

[3. Relation between Head Gravity Center Distance Db and Shaft Torque Ts]

In the head 4, the head gravity center distance Db is 37 mm or more. As the head gravity center distance Db is increased, a moment of inertia of the head around the shaft axis Z is increased, and directional stability of the hit ball is increased. In addition, the hitting point and the sweet spot SS easily coincide with each other, and rebounding performance can be improved.

Meanwhile, it was determined that the grasp of the ball gets worse as the head gravity center distance Db is increased.

The grasp of the ball is a concept related to a direction of a face upon impact. “Grasp of a ball is good” means that the face upon impact is not open and a ball can be caught firmly. “Grasp of a ball is bad” means that the face upon impact is open and the face fails to catch the ball. Grasp of a ball is also simply referred to as grasp.

As a shaft torque Ts is reduced, even when the head gravity center distance Db is increased, grasp is determined as good. When the following conditions are satisfied, a golf club having good grasp is obtained while enjoying an advantage that the head gravity center distance Db is large.

(a) The head gravity center distance Db is 37 mm or more.

(b) The shaft torque Ts is less than 4.0°.

(c) Db/Ts is 9.7 or more.

In view of directional stability and rebounding performance of a hit ball, the head gravity center distance Db is preferably 37 mm or more, more preferably 38 mm or more, and even more preferably 39 mm or more. Considering a limitation in a degree of design freedom, the head gravity center distance DU is preferably 45 mm or less, more preferably 43 mm or less, and even more preferably 41 mm or less.

In view of the grasp, the shaft torque Ts is preferably less than 4.0°, more preferably 3.9° or less, and further preferably 3.8° or less. In view of strength of the shaft, the shaft torque Ts is preferably 2.0° or more, more preferably 2.2° or more, and even more preferably 2.4° or more.

In view of providing good grasp while securing directional stability of the hit ball, Db/Ts is preferably 9.7 or more, more preferably 9.8 or more, even more preferably 9.9 or more, even more preferably 10.0 or more, even more preferably 10.1 or more, even more preferably 10.2 or more, even more preferably 10.3, even more preferably 10.4 or more, and even more preferably 10.5 or more. Considering a limitation in a degree of design freedom, Db/Ts is preferably 15.0 or less, more preferably 14.8 or less, and even more preferably 14.5 or less.

[4. Lg/Ls]

As described above, Lg is a distance from the center of gravity Gs of the shaft to the tip end Tp, and Ls is a shaft length. It was determined that grasp is further improved as Lg/Ls is increased. In an embodiment, Lg/Ls is as follows.

(d) Lg/Ls is 0.50 or more.

As Lg/Ls is increased, the center of gravity Gs of the shaft approaches the grip 8. In this case, it is possible to make the head 4 heavier while maintaining a normal swing weight. For this reason, it becomes easy for the head to run upon down swing, and the face is easily returned. In this case, grasp becomes good. In addition, because the head 4 can be heavier, rebounding performance is improved.

In view of grasp and rebounding performance, Lg/Ls is preferably 0.50 or more, more preferably 0.51 or more, and even more preferably 0.52 or more. Considering a limitation of a degree of design freedom, Lg/Ls is preferably 0.60 or less, more preferably 0.59 or less, and even more preferably 0.58 or less.

In view of easiness of swing, a swing weight (a 14 inch type) is preferably D5 or less, more preferably D3 or less, and even more particularly D1 or less. When the head weight is large, rebounding performance is improved. In view of this, the swing weight is preferably C7 or more, more preferably C8 or more, and even more preferably C9 or more.

[5. Relation between Club Weight Wc and Club Length Lc]

Easiness of swing is secured and a head speed is increased and a driving distance is lengthened by a golf club having a relatively small weight with respect to a club length Lc. In an embodiment, a club weight Wc (grams) and the club length Lc (meters) satisfy the following relation.

(e) Wc×Lc<360 (g·m)

In the golf club having Wc×Lc of 360 (g·m) or less, a swing speed is increased. For this reason, a time required for the down swing is reduced, and an impact is easily received without returning the face. That is, while the head speed is increased, the grasp is easily degraded. Since Db/Ts is 9.7 or more, even when Wc×Lc is 360 (g·m) or less, grasp becomes good.

In view of the above, Wc×Lc is preferably 360 (g·m) or less, more preferably 350 (g·m) or less, and even more preferably 345 (g·m) or less. Considering a limitation of a degree of design freedom, Wc×Lc is preferably 310 (g·m) or more, more preferably 320 (g·m) or more, and even more preferably 325 (g·m) or more.

In view of minimization of Wc×Lc, the weight of the shaft is preferably 54 g or less, more preferably 52 g or less, even more preferably 50 g or less, and even more preferably less than 50 g. In view of the degree of design freedom of the shaft, the shaft weight is preferably 40 g or more, more preferably 42 g or more, and even more preferably 44 g or more.

[6. Laminated Constitution and Club Performance of Shaft]

The laminated constitution of the shaft contributes to an increase in performance of a golf club.

[6-1. Bias Layer of Tip Portion and Bias Layer of Butt Portion]

The shaft torque can be effectively reduced while minimizing the weight of the entire bias layer by localizing the bias layer on the tip portion and the butt portion. As a result, a shaft that can be easily swung with a light weight and having good grasp is obtained.

In view of reduction in a shaft torque, the bias layer is disposed on the tip portion according to an embodiment. Grasp is easily affected by twist rigidity of a portion close to the grip (a golfer's hand), in addition to proximity to the head. As the bias layer is distributed on the tip portion and the butt portion, grasp is improved while minimizing the weight of the bias layer (a bias distribution effect).

In addition, in the embodiment, the weight of the bias layer of the butt portion is larger than that of the bias layer of the tip portion. For this reason, Lg/Ls can be increased while enjoying the bias distribution effect.

[6-2. Fibers of Straight Layer of Butt Portion]

The above-mentioned layer s5 is a straight layer of the butt portion. The straight layer of the butt portion may not be provided. When the straight layer of the butt portion is formed, a carbon fiber and a glass fiber are exemplified as fibers of the straight layer of the butt portion. In the embodiment, the straight layer of the butt portion is a glass fiber reinforcement layer. A specific weight of the glass fiber is larger than that of the carbon fiber. In view of an increase in Lg/Ls, the straight layer of the butt portion is a glass fiber reinforcement layer according to an embodiment.

[6-3. Fibers of Straight Layer of Tip Portion]

The above-mentioned layer s9 is the straight layer of the tip portion. The layer s9 is the straight layer of the tip portion that is not the outermost layer. The straight layer s9 of the tip portion is a glass fiber reinforcement layer. The glass fiber reinforcement layer has high impact absorbing energy on the tip portion.

[6-4. Glass Fiber Reinforcement Layer of Tip Portion and Butt Portion]

As the glass fiber reinforcement layer having a large specific weight is disposed on the tip portion and the butt portion, a moment of inertia of the shaft around the center of gravity Gs of the shaft is increased. Accordingly, a behavior of the shaft during a swing is stabilized. Further, the weight of the glass fiber reinforcement layer s5 of the butt portion is larger than that of the glass fiber reinforcement layer s9 of the tip portion. This contributes to an increase in Lg/Ls.

[6-5. Partial Tip Layer and Butt Partial Layer]

In the embodiment of FIG. 3, the bias layers s3 and s4 of the butt portion, the straight layer s5 of the butt portion, the bias layers s10 and s11 of the tip portion, and the straight layers s9 and s12 of the tip portion are formed. As a weight distribution to both end portions of the shaft is increased, a moment of inertia of the shaft around the center of gravity Gs of the shaft is increased. For this reason, behavior of the shaft during a swing is stabilized. As a result, a variation in a hit ball direction is minimized.

[7. Measure Method]

A measurement method is as follows.

[7-1. Shaft Torque Ts]

FIG. 4 shows a measurement method of a shaft torque. A portion from a point of 40 mm from the tip end Tp to the tip end Tp is fixed by a jig M1. The fixing is accomplished by a pneumatic chuck, and a pneumatic pressure of the pneumatic chuck is 2.0 kgf/cm². A jig M2 is fixed to a portion having a width of 50 mm from a position separated by 750 mm from the jig M1. The fixing is accomplished by the pneumatic chuck, and a pneumatic pressure of the pneumatic chuck is 1.5 kgf/cm². A torque of 13.9 kg·cm was applied to the shaft 6 by rotating the jig M2 while fixing the jig M1. A twist angle due to the torque is a shaft torque (°).

[7-2. Club Length Lc]

A sole of the club abuts the ground plane according to a lie angle of the club. A point having a distance of 0.625 inch from the ground plate, among points on an intersection line between a plane including the shaft axis Z and perpendicular to the ground plane and an outer surface of the head on the side of the sole is a reference point k. An axial distance between the reference point k and an edge of the grip on the side of the butt is a club length Lc.

[7-3. Swing Weight (14 Inch Balance)]

The swing weight is measured using a trade name “BANCER-14” from DAININ Co., Ltd. The swing weight is 14 inch balance.

The swing weight is expressed by a mark obtained by combining an alphabet and a numeral. The alphabet is one of A to F. The numeral is an integer of 0 to 9. Further, numbers below a decimal point are rounded off In the swing weight, a position spaced 14 inches from a grip end is a support point. A swing weight is determined on the basis of a numerical value at which a club weight (ounce) is applied to an axial distance (inch) from the support point to a center of gravity of the club. The numerical value is divided into six steps of A to F. Further, A to F are subdivided by numerical values of 0 to 9. The swing weight is increased as it goes from A toward F, and the swing weight is increased as the numerical value is increased.

[8. Example of Usable Prepreg]

Examples of a prepreg used in the shaft of the disclosure are shown in the following Tables 1 and 2. Specification of the shaft can be adjusted according to selection of these various materials.

TABLE 1 Examples of usable prepregs Reinforcement fiber physical properties Thickness Fiber Resin Fiber Modulus of Tensile of sheet contents contents Product elasticity strength Manufacturer Trade Name (mm) (wt %) (wt %) No. (t/mm²) (kgf/mm²) Toray 3255S-10 0.082 76 24 T700S 24 500 Toray 3255S-12 0.103 76 24 T700S 24 500 Toray 3255S-15 0.123 76 24 T700S 24 500 Toray 2255S-10 0.082 76 24 T800S 30 600 Toray 2255S-12 0.102 76 24 T800S 30 600 Toray 2255S-15 0.123 76 24 T800S 30 600 Toray 2256S-10 0.077 80 20 T800S 30 600 Toray 2256S-12 0.103 80 20 T800S 30 600 Toray 2276S-10 0.077 80 20 T800S 30 600 Toray 805S-3 0.034 60 40 M30S 30 560 Toray 8053S-3 0.028 70 30 M30S 30 560 Toray 9255S-7A 0.056 78 22 M40S 40 470 Toray 9255S-6A 0.047 76 24 M40S 40 470 Toray 925AS-4C 0.038 65 35 M40S 40 470 Toray 9053S-4 0.027 70 30 M40S 40 470 Nippon Graphite E1026A-09N 0.100 63 37 XN-10 10 190 Fiber Corp. Nippon Graphite E1026A-14N 0.150 63 37 XN-10 10 190 Fiber Corp. Tensile strength and modulus of elasticity are values obtained through measurement pursuant to JIS R7601: 1986 “Carbon Fiber Test Method.”

TABLE 2 Examples of usable prepregs Reinforcement fiber physical properties Thickness Fiber Resin Fiber Modulus of Tensile of sheet contents contents Product elasticity strength Manufacturer Trade Name (mm) (wt %) (wt %) No. (t/mm²) (kgf/mm²) Mitsubishi GE352H-160S 0.150 65 35 E Glass 7 320 Rayon Mitsubishi TR350C-100S 0.083 75 25 TR50S 24 500 Rayon Mitsubishi TR350U-100S 0.078 75 25 TR50S 24 500 Rayon Mitsubishi TR350C-125S 0.104 75 25 TR50S 24 500 Rayon Mitsubishi TR350C-150S 0.124 75 25 TR540S 24 500 Rayon Mitsubishi TR350C-175S 0.147 75 25 TR50S 24 500 Rayon Mitsubishi MR350J-025S 0.034 63 37 MR40 30 450 Rayon Mitsubishi MR0350J-050S 0.058 63 37 MR40 30 450 Rayon Mitsubishi MR350C-050S 0.05 75 25 MR40 30 450 Rayon Mitsubishi MR350C-075S 0.063 75 25 MR40 30 450 Rayon Mitsubishi MRX350C-075R 0.063 75 25 MR40 30 450 Rayon Mitsubishi MRX350C-100S 0.085 75 25 MR40 30 450 Rayon Mitsubishi MR350C-100S 0.085 75 25 MR40 30 450 Rayon Mitsubishi MRX350C-125S 0.105 75 25 MR40 30 450 Rayon Mitsubishi MR350C-125S 0.105 75 25 MR40 30 450 Rayon Mitsubishi MR350E-100S 0.093 70 30 MR40 30 450 Rayon Mitsubishi HRX350C-075S 0.057 75 25 HR40 40 450 Rayon Mitsubishi HRX350C-110S 0.082 75 25 HR40 40 450 Rayon Tensile strength and modulus of elasticity are values obtained through measurement pursuant to JIS R7601: 1986 “Carbon Fiber Test Method.”

EXAMPLES

Hereinafter, while the effect in this disclosure will be apparent by examples, the disclosure will not be restrictively selected on the basis of description of the examples.

Example 1

A shaft for a number 6 iron was fabricated through the above-mentioned processes. The laminated constitution is the same as shown in FIG. 3. The straight layer s5 of the butt portion and the straight layer s9 of the tip portion were the glass fiber reinforcement layers, and the other layers were the carbon fiber reinforcement layers. A grip and a head of the number 6 iron were mounted on the shaft, and a golf club of Example 1 was obtained. Specifications and estimated results of Example 1 are shown in the following Table 3.

Examples 2 to 7 and Comparative Examples 1 and 2

Golf clubs of Examples 2 to 7 and Comparative examples 1 and 2 were obtained in the same manner as in Example 1 except for the specifications shown in the following tables. The specifications and estimated results are shown in the following Tables 3 and 4.

Further, in all of the examples and the comparative examples, a swing weight (14 inch type) is D0. A head weight or the like was adjusted such that the swing weight is D0. In Examples 1 to 4, since Lg/Ls was varied, the head weight was adjusted and the swing weight was D0. In Examples 5 and 6, the shaft weight, the grip weight and the head weight were adjusted, and the swing weight was D0.

In Comparative example 2, the bias layers s3 and s4 of the butt portion and the bias layers s10 and s11 of the tip portion were not used. Instead of using the layers s3, s4, s10 and s11, the weight of the full length bias layers s1 and s2 was increased, and the weight of the bias layer was equal to that of Example 1. Accordingly, in Comparative example 2, weight distribution of the shaft to the tip portion and the butt portion is smaller than other examples and comparative examples.

TABLE 3 Specifications and estimated results of Examples Unit Example 1 Example 2 Example 3 Example 4 Shaft weight gram 48 48 48 48 Shaft length Ls mm 934 934 934 934 Distance Lg mm 486 479 469 456 Lg/Ls — 0.52 0.51 0.50 0.49 Shaft torque Ts degree 3.7 3.7 3.7 3.7 Head gravity center distance mm 38.4 38.4 38.4 38.4 Db Db/Ts — 10.4 10.4 10.4 10.4 Club weight Wc gram 356 354 352 350 Club length Lc m 0.957 0.957 0.957 0.957 Wc × Lc g · m 341 339 337 335 Swing weight — D0 D0 D0 D0 Presence of partial bias layer — Presence Presence Presence Presence of tip portion and butt portion Deviation in hit ball direction yard −1.1 +2.2 +4.1 +7.8 Deviation width yard 4.8 5.2 6.3 6.7 Driving distance yard 140 138 137 135

TABLE 4 Specifications and estimated results of Examples and Comparative examples Comparative Comparative Unit Example 5 Example 6 Example 7 example 1 example 2 Shaft weight gram 58 68 48 48 48 Shaft length Ls mm 934 934 934 934 934 Distance Lg mm 486 486 486 486 486 Lg/Ls — 0.52 0.52 0.52 0.52 0.52 Shaft torque Ts degree 3.7 3.7 3.9 3.7 4.3 Head gravity center mm 38.4 38.4 38.4 35.0 40.2 distance Db Db/Ts — 10.4 10.4 9.8 9.5 9.3 Club weight Wc gram 366 377 356 356 356 Club length Lc m 0.957 0.957 0.957 0.957 0.957 Wc × Lc g · m 350 361 341 341 341 Swing weight — D0 D0 D0 D0 D0 Presence of partial — Presence Presence Presence Presence None bias layer of tip portion and butt portion Deviation in hit yard +1.9 +0.4 +3.8 −0.6 +12.9 ball direction Deviation width yard 4.6 4.5 5.8 13.7 14.6 Driving distance yard 134 130 136 125 127

The estimation method is as follows.

[Deviation of Hit Ball Direction]

Ten right-handed testers did actual shot tests. A distance (yard) of a deviation from a target direction was measured. The distance was positive when a rightward deviation occurs rightward, and the distance was negative when a leftward deviation occurs. It means that grasp gets worse as the deviation goes rightward. Ten testers hit five balls each club. An average value of 50 pieces of data is shown in a row of “a deviation of a hit ball direction.”

[Deviation Width]

In the actual shot data, among five pieces of data of each tester, an absolute value of the deviation of the hit ball that moved to the rightmost side and an absolute value of the deviation of the hit ball that moved to the leftmost side are summed. An average value of the sum is shown in a row of “a deviation width.” As the deviation width is increased, a variation in hit ball direction is large.

[Driving Distance]

In the actual shot test, a driving distance was also measured. A driving distance of a final arrival point was measured. An average value of 50 pieces of data is shown in a row of “a driving distance.”

As shown in Table 3 and Table 4, Examples have better estimation than Comparative examples. From the estimation results, advantages of the disclosure are obvious.

The disclosure may be applied to the iron golf club.

It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed embodiments without departing from the scope or spirit of the disclosure. In view of the foregoing, it is intended that the disclosure covers modifications and variations provided that they fall within the scope of the following claims and their equivalents. 

What is claimed is:
 1. An iron golf club, comprising: a head, a shaft and a grip, wherein a head gravity center distance that is a distance between a center of gravity of the head and an axis of the shaft is Db (mm), and when a shaft torque of the shaft is Ts (°), Db is 37 mm or more, Ts is less than 4.0°, and Db/Ts is 9.7 or more.
 2. The golf club according to claim 1, wherein, provided that a distance from a center of gravity of the shaft to a tip end of the shaft is Lg and a length of the shaft is Ls, Lg/Ls is 0.50 or more.
 3. The golf club according to claim 1, wherein, provided that a club weight is Wc (grams) and a club length is Lc (meters), Wc×Lc is 360 (g·m) or less.
 4. The golf club according to claim 1, wherein the shaft is Ruined of a plurality of fiber reinforcement layers, and the shaft has a bias layer of a tip portion and a bias layer of a butt portion.
 5. The golf club according to claim 2, wherein, provided that a club weight is Wc (grams) and a club length is Lc (meters), Wc×Lc is 360 (g·m) or less.
 6. The golf club according to claim 2, wherein the shaft is formed of a plurality of fiber reinforcement layers, and the shaft has a bias layer of a tip portion and a bias layer of a butt portion.
 7. The golf club according to claim 3, wherein the shaft is formed of a plurality of fiber reinforcement layers, and the shaft has a bias layer of a tip portion and a bias layer of a butt portion. 